This invention relates to a gas BTU measurement apparatus and method, and more particularly, to the continuous measurement of the BTU content of natural gas using a microcalorimeter .mu.-sensor system.
The BTU content of natural gas typically varies between 900 and 1200 BTU/Ft.sup.3. Methane content can range from a low of 80% up to close to 100%. Precise measurement of the BTU content of natural gas is extremely important in a wide variety of applications. Current methods are time consuming and use expensive equipment. A simple, low-cost, and reliable microsensor (.mu.-sensor) that can measure the BTU content of a hydrocarbon stream with high accuracy is highly desirable.
Gas sensors for the measurement of the lower explosion limit of combustible gases like methane (such as in safety alarm devices, also called flammable gas alarms) have been available for many years. Most of the modern combustible gas sensors are built like tiny heterogeneous catalytic reactors. A schematic of such a combustible gas sensor is shown in FIG. 1. The sensor consists of a coil of fine platinum (Pt) wire over which a catalyst (Pt on alumina) bead has been formed. The overall dimensions of the bead can be much less than a mm.sup.3 making it a very small sensor indeed.
In such gas sensors, the bead is heated by passing a current through the platinum wire. Any combustible gas that comes into contact with the hot catalyst on the bead surface reacts according to equation 1 below (a similar reaction can be written for any hydrocarbon), producing some heat, denoted dH. The amount of heat produced is proportional to the heat of combustion of the hydrocarbon and the number of molecules reacting (concentration). EQU CH.sub.4 +2 O.sub.2 =CO.sub.2 +2 H.sub.2 O+dH (1)
or more generally EQU a [HC]+b O.sub.2 =c CO.sub.2 +d H.sub.2 O+dH (2)
The platinum wire in the sensor is also a resistance thermometer, i.e., its resistance changes with temperature. Some of the heat (dH) from the combustible gas reaction at the catalyst surface is lost by conduction, convection, and radiation, while some part of the dH goes into the catalyst bead causing an increase in bead temperature (dH=C.sub.p dT). The change in bead temperature, in turn, causes a change in resistance of the sensor's Pt wire. The change in resistance of the Pt wire is typically monitored by placing the bead in a Wheatstone bridge circuit with a compensating element (passivated and matched) and two known resistors. Small changes in temperature corresponding to a few hundred to a few thousand ppm of CH.sub.4 can be detected as an imbalance in the resistance bridge circuit. This relationship is usually expressed as EQU S=K dH [CH.sub.4 ] (3)
where S is the "signal" from the sensor (corresponding to the imbalance in the bridge in volts) and K is an instrument constant that is obtained by calibration of the sensor with a known concentration of methane in air.
The signal from combustible gas sensors wherein the gas concentration is measured is typically linear over the range 0-5% methane for some sensors and linear up to 10% methane for others. Because the sensor is operated in a diffusion limited region, the signal is proportional to the amount of gas reaching the bead.
Since the total amount of gas diffusing to the bead is constant, the output signal depends upon the relative amount of methane in the sample. The constant, K, depends upon the type of gas, i.e., methane, ethane, or propane, being detected (i.e., different gases cause a different temperature rise) and the sensor housing design (the geometry of the device). The value of the constant K varies with the type of gas for two reasons: (1) different gases have different heat content and therefore produce more or less heat upon combustion, and thus produce more or less of a temperature rise to be transduced from a thermal into an electrical signal, and (2) the calorimetric or heat transfer properties of the bead and gas can change from bead to bead, from gas to gas, and from method to method.
As discussed below, one aspect of the present invention concerns the use of microprocessor control for injection of a gas sample. When samples are injected manually, this necessarily involves a start and stop procedure. The results therefore, are never as precise as a computer controlled system. A computer operating, for example, with a 1 microsecond sampling time is able to perform operations with a theoretical precision in the order of one part in a million. Further, computer control will cause a switch (a solenoid control valve for the controlled injection of a sample) to always have a constant response time, e.g. 3 millisecond. Hence, the flow perturbations, pressure perturbations and temperature perturbations that are introduced by these fast operations are practically nil, and cannot be sensed by a sensor.
To the contrary, in a manually operated system, performing a function may take a couple of seconds, for example, which introduces fluctuations large enough that they would disturb the measuring system so that error is introduced.
Calorimeters are currently available, such as ones manufactured by Cutler-Hammer, that measure BTU content to a high degree of precision, such as in the &lt;1% range. These devices, however, are expensive, costing around $30,000, use a flame rather than a catalytic sensor, and do not have the portability that is highly desired. No devices presently known in the art use a catalytic sensor to produce BTU measurement with accuracies &lt;1%, and provide convenient portability, at low production cost.